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We ve seen requirements common to many applications and how these can be met without recoding the same stuff the company across the street did last week. We ve discussed the importance of keeping business logic uncluttered with cross-cutting concerns. And we ve cringed at the unnecessary plumbing employed by the roll-your-own approach. Most importantly, we ve revealed the EJB Specification as a viable solution to: Address common/generic issues within application development Code less Standardize Integrate with other technologies under the umbrella of the Java Enterprise Edition
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The specification offers a few different bean types, and the most sensible choice will depend upon the intended purpose.
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Modeling real-life objects and concepts is one of the first skills a programmer must develop. As such, we ve become fairly adept at implementing object-oriented axioms such as reusability and extensibility on a daily basis. When we focus these principles on business logic, we end up with a set of business objects that encapsulate the rules of the road. Starting with the 3.0 version of the Specification, EJB imposes no API coupling or restrictions upon the classes that will define our business objects. Commonly known as POJO (Plain Old Java Object)* development, this means that an application developer is under no obligation to extend, implement, or have any references tying the application to EJB. Now we can create class hierarchies however we see fit, and reuse our objects in some non-EJB environment (perhaps for quick testing outside of the Container). Because a POJO class is just like any other class, it does not become an EJB until it s: 1. Assembled/packaged 2. Deployed 3. Accessed via the Container This is an important distinction. EJBs become such only in the context of the EJB Container. The Container, in turn, is responsible for equipping POJOs with EJB Services (covered in 3) as well as exposing their behavior via one of three personalities. We call these personalities component types, and while implementing their semantics is beyond scope for an application developer, it s important to know how, by contract, they ll behave. We ll use the example of a fictitious casino to showcase where each component type might be applied.
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* Fowler et al. http://www.martinfowler.com/bliki/POJO.html
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Server-side component types reside exclusively on the server, and the client must interact with them via some indirection. There are two major server-side component types: session beans, which expose a view for the client to invoke upon, and messagedriven beans, which act as event listeners.
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If EJB is a grammar, session beans are the verbs. They take action, and they frequently contain business methods. Because of EJB s distributed nature, underlying bean instances that carry out the invocation live on the server and are accessed by way of a simple view the client may request of the EJB Container. This means that the client does not access the EJB directly, which allows the Container to perform all sorts of magic before a request finally hits the target method. It s this separation that allows for the client to be completely unaware of the location of the server, concurrency policies, or queuing of requests to manage resources. As far as the client is concerned, it s operating directly upon an EJB. In truth, the client is invoking upon a proxy reference that will delegate the request along to the Container and return the appropriate response (see Figure 2-1).
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Ultimately, it s the bean instances created and managed by the Container that the service client requests.
Stateless session beans (SLSBs)
Stateless session beans are useful for functions in which state does not need to be carried from invocation to invocation. A client cannot assume that subsequent requests will
target any particular bean instance. In fact, the Container will often create and destroy instances however it feels will be most efficient (see Figure 2-2). How a Container chooses the target instance is left to the vendor s discretion.
Because there s no rule linking an invocation to a particular target bean instance, these instances may be used interchangeably and shared by many clients. This allows the Container to hold a much smaller number of objects in service, hence keeping memory footprint down. One caveat to beware: though a SLSB has stateless semantics, it s backed by an instance of a class created by the application developer. Particular care must be employed to assure that any shared members (instance variables, for instance) are not leaked between invocations; this may lead to unpredictable behavior. We ll explain this gotcha when we cover SLSBs in greater detail in 5. If we were to give our casino a game of roulette, SLSB would be a natural implementation choice. Roulette is a game with no memory each spin operates independently from the last so a function getSpinResult should return a random spot on the wheel.
Stateful session beans (SFSBs)
Stateful session beans differ from SLSBs in that every request upon a given proxy reference is guaranteed to ultimately invoke upon the same bean instance. This is to say, SFSB invocations share conversational state. Each SFSB proxy object has an isolated session context, so calls to one session will not affect another. Stateful sessions, and their corresponding bean instances, are created sometime before the first invocation upon a proxy is made to its target instance (Figure 2-3). They live until the client invokes a method that the bean provider has marked as a remove event, or until the Container decides to evict the session (usually due to some timeout, though the spec leaves this out-of-scope and up to the vendor).
In order to minimize the number of stateful sessions carried around in memory, the Container may passivate a SFSB bean instance. During passivation, the session s state is flushed to some persistent storage such that it may be removed from RAM. If the session is needed again before it s removed for good, the Container will activate it and bring the bean instance back into memory. No casino is complete without a good game of poker, and we could build ours using SFSBs. Each game is played using a single deck, and we need to keep track of which cards have already been dealt otherwise we risk giving the player next to us the impossible hand of five aces! If each table is scoped to its own stateful session, we could ensure that the integrity of the deck is intact. We ll dive into SFSBs in 6.
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